1. Field of the Invention
The present invention generally relates to data transmission in a communication system, and more specifically to acoustic data transmission involving an ingestible capsule.
2. Related Art
The surface between the animal body and air is almost a perfect acoustic reflector. Therefore, the animal body is an acoustic reverberation chamber, where sound launched into and within the body echoes back and forth between these surfaces until attenuation causes the sound to die out. Other materials in the body such as lungs, gas pockets, bone, etc., cause reflections which further add to the effect of a reverberation chamber. Attenuation is linearly dependent upon frequency, with higher frequencies having greater attenuation. For example, a 500 KHz acoustic pulse launched within the body will take almost 200 microseconds for the echoing to die out, while a 1 MHz pulse would take only 100 microseconds, and a 100 KHz pulse echo would not die out until a millisecond has passed.
The body is not a static fixed cavity resonator, but rather is a dynamic one, with the echo characteristics changing with time. Many factors affect the dynamic behavior of this reverberation chamber, including breathing, the heart beat, speaking, organ movement, bowel function, vein pulsing, body movement, and even Doppler frequency shifting. The result is that an acoustic signal source (modulation at a given frequency or set of frequencies) in the body will create a noise signal which is a complicated function of the multiple echoes of all previously sent frequencies and which is amplitude modulated and phase shifted by the differences in tissue densities and dynamic changes of the body cavity. A modulation is a change to a carrier frequency, which includes a change from a constant wave to a reduced amplitude constant wave as is used typically to transfer both power and data communications. The complex noise signal in the acoustic communication channel makes high data rate information transfer a difficult challenge within the body as it is difficult for the receiver, attached to the skin of the body, to distinguish a signal transmitted by an ingestible diagnostic capsule from this additional noise signal that will accompany the intended signal. Low data rates are achieved simply by waiting until the noise dies out before sending another data bit or symbol which can be unambiguously identified by the receiver. However, to achieve high data rates, data bits or symbols need to be pushed through the channel in the presence of the noise. Therefore, what is needed is a method and apparatus that may achieve high data rates by pushing data bits or symbols through the channel in the presence of the noise.